Tetraphenylethene-Induced Free Volumes for the Isomerization of Spiropyran toward Multifunctional Materials in the Solid State

Solid-state
photochromic materials with reversible and adjustable
optical properties are very appealing because of their wide prospects
in advanced functional materials. Yet, it remains a significant challenge
to develop such materials in the solid state. In this study, a tetraphenylethene
derivative (SP–TPE–SP)-based solid-state
photoswitch, which exhibits reversible photochromism in the solid
state, was constructed. Efficient photoswitching between SP–TPE–SP and its photoisomer MC–TPE–MC is assisted
by the large free volumes caused by the nonplanar molecular structures
of the TPE1 moieties and the intramolecular π–π
stackings between the two MC moieties. The free volumes are large
enough to allow for the transport of HCl gas molecules for an acidochromic
response. Furthermore, the morphology of the SP–TPE–SP solid surface can be regulated by ultraviolet light irradiation.
The contact angles of the SP–TPE–SP solid
surface can be decreased, changing from 97 to 82°. Therefore, SP–TPE–SP with a rather simple molecular structure
is appealing for advanced multifunctional materials.